Sometimes it feels like spilling coffee from your mug is inevitable. In fact, it turns out that hunch might be correct: scientists have shown that humans are biologically built to spill.

In what sounds like incredibly frivolous research, a team of mechanical engineers from the University of California, Santa Barbara, has been investigating why people spill coffee. To do that, they had volunteers carry cups of the stuff whilst walking in a straight line at different speeds. A camera recorded the person's motion and the mug's trajectory, and a small sensor on the mug recorded the instant of spillage.

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The team hit on a curious finding, which they've published in Physical Review E. You see, the back-and-forth motion of the fluid in most conventional-sized mugs shares a natural frequency that just happens to match that of the average person's leg movements during walking. Basically, the rate at which we move our legs when we walk naturally exacerbates the swilling of coffee in the cups we tend to use, making us more likely to spill.

The researchers also found that small irregularities in a person's walking can amplify the wilder oscillations which are set up by the naturally-matched step and slosh, further increasing the chances of a spillage.

So what can you do to avoid a spill? The obvious choice is to move mug size: in particular, something much narrower will cause fewer accidents—but to consume the same amount of coffee, you would need a very, very tall mug, Instead, the researchers suggest starting slow and gradually building pace as you walk, to avoid generating oscillations because of a sharp increase in momentum. Or, you could just put less coffee in your cup. With nature already against you, the last thing you need is the jitters.[Physical Review E via Science]